This invention relates to the determination of the maximum temperature that an article reaches during testing or service, and, more particularly, to a method for making such a determination from the structure of the material.
Many metallic articles are heated to a range of temperatures during use, either intentionally or due to some unforeseen event. In many situations, the maximum temperature reached by the article is of critical concern, while in others the accumulated time at lower temperatures can be of primary interest. The maximum temperature may be the principal determinant of the life of the article, due to phase changes or other phenomena that occur very rapidly after the article reaches that maximum temperature.
One important materials application where maximum temperature is a key concern is the hot section components of aircraft gas turbine (jet) engines. In an aircraft gas turbine engine, air is drawn into the intake of the engine and compressed. Fuel is added to the compressed air, and the mixture is burned to produce exhaust gas. The exhaust gas passes through the hot section of the engine, which includes turbine vanes that bend the gas flow direction slightly and turbine blades mounted on a rotatable disk. The impact of the exhaust gas flow against the turbine blades causes the turbine disk to rotate, which causes a shaft to rotate. The shaft runs up the center of the engine to the compressor, and provides the driving force for operating the compressor.
The turbine vanes, turbine blades, and other hot-section component are desirably made from a nickel-based superalloy. Such materials have maximum operating temperatures of about 2000.degree.-2250.degree. F., depending upon the composition of the metal and the manner in which it is used. A critical concern is the maximum temperature reached by the hot-section component during either testing or service, because of temperature-dependent phase changes and high-temperature failure mechanisms. Brief excursions to elevated temperature often occur in an engine during an emergency-power situation. Those who design and build the engines must know the temperature reached to within a few degrees accuracy so that they can evaluate the effect of the temperature excursion on engine life and performance.
There have been many approaches to the measurement of the maximum temperature reached in the hot sections of the gas turbine engine. Thermocouples, infrared sensors, and other types of temperature measurement devices have been utilized. Such sensors may be operable for the stationary components of the engine, such as the turbine vanes, but lack durability and versatility for the rotating components such as the turbine blades that rotate at rates of 30,000 revolutions per minute or more.
As an alternative temperature-measurement approach, particularly for the rotating components, various metallurgical techniques have been used to estimate the temperature reached by the article. For example, coatings are sometimes applied to the turbine blades to protect them in the hot exhaust gas stream. The degree of wrinkling, melting, or interdiffusion of the coating with the substrate may be used to estimate the maximum temperature reached. Another approach involves measuring alloy, carbide, or subcoating sigma phase dissolution as an indicator. Sigma phase is a deleterious, brittle phase which adversely affects creep/rupture strength. Each of these techniques has drawbacks In most instances, the technique measures not the maximum temperature reached, but some integrated function of time and temperature experienced by the article. In other instances, the technique is completely inoperable for some reason, such as the absence of a phase entirely. No reliable technique for measuring the maximum temperature reached by the article is available.
There is therefore a need for a technique for determining the maximum temperature reached by an article, such as a nickel-based superalloy article used in the hot section of an aircraft gas turbine engine. The technique must be reasonably accurate, and operable for rotating components. The present invention fulfills this need, and further provides related advantages.